Modacrylic fibers are composed of modified acrylic fibers made from acrylonitrile groups, often as copolymers made with some amount of other polymer. The Federal Trade Commission defines modacrylic fibers as manufactured fibers in which the fiber-forming substance is any long chain synthetic polymer composed of less than 85%, but at least 35 weight percent acrylonitrile units. 16 CFR § 303.7 (b). Modacrylics have many desirable properties, including strength and resilience, resistance to chemicals and solvents, resistance to insects and mildew, quick drying, non-allergenic, good shape retention, flame resistance, and are soft to the touch. Modacrylic fibers are often combined with cellulosic fibers to produce flame resistant modacrylic/cellulosic blend fabrics.
Vat dyes are a group of dyes which are not water soluble in their normal form. Vat dyes are made to be water soluble by the chemical process of reduction, usually with a reducing agent such as sodium hydrosulfite and an alkali such as sodium hydroxide. The reduced form of the dye is known as the lueco form. Vat dyes have high wash fastness, are usually very lightfast, and are resistant to chlorine bleach. Vat dyes are typically used for garments and other items which will be subjected to multiple washings. Vat dyes require several complex chemical steps, and control of appropriate conditions. Due to various reasons, including the complex dyeing process, vat dyes are used more with continuous dyeing than in exhaust processing. Vat dyes are particularly well suited for dyeing cotton fibers. In the lueco form, the vat dye is highly water-soluble and is highly attracted to the cotton fiber where it rapidly absorbs into the fiber structure. The vat dyes are then oxidized to return them to the insoluble form, resulting in the vat dye being secured within the cotton fiber. Vat dyes are highly resistant to chlorine and exhibit good color fastness properties on cotton fiber.
Vat dyes are typically not used for dyeing modacrylics. A class of dyes known as basics or cationics is used because these dyes provide a cationic, positively charged dye molecule to link with an anionic negatively charged dye site on the modacrylic fiber. A typical dye bath for modacrylics is acidic comprising an acid such as acetic acid (pH 3.5-5.0). Also, modacrylics are not very absorbent. Accordingly, vat dyes are not useful for dyeing modacrylic/cellulosic blend fabrics because, while the cellulosic fibers will readily take up the dye, the modacrylic fibers generally will not. This results in uneven dyeing and an undesirable result.
There are two broad methods for dyeing textiles; continuous and exhaust or batch dyeing. Continuous dyeing methods employ a dye range with equipment to apply, process and dry fabrics. On a continuous dye range fabric may be dyed without stopping, as a new roll of fabric is sewn to the end of a previous roll so that the range does not have to be stopped. Exhaust dyeing is a batch process wherein fabric to be dyed is loaded into a vessel for dyeing, water, dye and chemicals are added to the vessel by a specific recipe, the bath heated, rinsed and washed before the vessel is unloaded. The wet, dyed fabric is taken to another machine for drying. The basic or cationic dyes conventionally used for dyeing modacrylic fibers do not lend themselves to dyeing by the continuous method, so modacrylic fabrics are traditionally dyed by a batch process. The invention allows for modacrylic-containing fabrics to be dyed by the continuous method, using vat dyes that are well suited to this method of application.